In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at sNN=2.42 GeV collected with HADES. In this energy regime the ...population of Δ(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dMπ±pdpTdy distributions of correlated π±p pairs have been determined by subtracting the πp combinatorial background using an iterative method. The invariant-mass distributions in the Δ(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1<Minv(GeV/c2)<1.4, the yield is found to be similar for π+p and π−p pairs, and to follow a power law 〈Apart〉α, where 〈Apart〉 is the mean number of participating nucleons. The exponent α depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is α=1.5±0.08st±0.2sy.
Hadron production (
π
±
, proton,
Λ
,
K
S
0
,
K
±
) in
π
-
+
C
and
π
-
+
W
collisions is investigated at an incident pion beam momentum of
1.7
GeV
/
c
. This comprehensive set of data measured with ...HADES at SIS18/GSI significantly extends the existing world data on hadron production in pion induced reactions and provides a new reference for models that are commonly used for the interpretation of heavy-ion collisions. The measured inclusive differential production cross-sections are compared with state-of-the-art transport model (GiBUU, SMASH) calculations. The (semi-) exclusive channel
π
-
+
A
→
Λ
+
K
S
0
+
X
, in which the kinematics of the strange hadrons are correlated, is also investigated and compared to a model calculation. Agreement and remaining tensions between data and the current version of the considered transport models are discussed.
.
The HADES data from p + Nb collisions at a center-of-mass energy of
GeV are analyzed employing a statistical hadronization model. The model can successfully describe the production yields of the ...identified hadrons
,
,
,
K
0
s
,
with parameters
MeV and
MeV, which fit well into the chemical freeze-out systematics found in heavy-ion collisions. In addition, we reanalyze our previous HADES data from Ar + KCl collisions at
GeV with an updated version of the model. We address equilibration in heavy-ion collisions by testing two aspects: the description of yields and the regularity of freeze-out parameters from a statistical model fit as a function of colliding energy and system size. Despite its success, the model fails to describe the observed
yields in both, p + Nb and Ar + KCl . Special emphasis is put on feed-down contributions from higher-lying resonance states as a possible explanation for the observed excess.
A feasibility study has been performed in order to investigate the performance of the HADES detector to measure the electromagnetic decays of the hyperon resonances
Σ
(
1385
)
0
,
Λ
(
1405
)
and
...Λ
(
1520
)
as well as the production of double strange baryon systems
Ξ
-
and
Λ
Λ
in p + p reactions at a beam kinetic energy of
4.5
GeV
. The existing HADES detector will be upgraded by a new Forward Detector, which extends the detector acceptance into a range of polar angles that plays a crucial role for these investigations. The analysis of each channel is preceded by a consideration of the production cross-sections. Afterwards the expected signal count rates using a target consisting of either liquid hydrogen or polyethylene are summarized.
Abstract The production of $$\Sigma ^{0}$$ Σ 0 hyperons in proton proton collisions at a beam kinetic energy of 3.5 GeV impinging on a liquid hydrogen target was investigated using data collected ...with the HADES setup. The total production cross section is found to be $${\sigma (pK^{+}\Sigma ^{0}) = 17.7 \pm 1.7 (stat) \pm 1.6 (syst)}$$ σ ( p K + Σ 0 ) = 17.7 ± 1.7 ( s t a t ) ± 1.6 ( s y s t ) µb. Differential cross section distributions of the exclusive channel $${pp \rightarrow pK^{+}\Sigma ^{0}}$$ p p → p K + Σ 0 were analyzed in the center-of-mass, Gottfried–Jackson and helicity reference frames for the first time at the excess energy of 556 MeV. The data support the interplay between pion and kaon exchange mechanisms and clearly demonstrate the contribution of interfering nucleon resonances decaying to $$\textrm{K}^{+}\Sigma ^{0}$$ K + Σ 0 . The Bonn–Gatchina partial wave analysis was employed to analyse the data. Due to the limited statistics, it was not possible to obtain an unambiguous determination of the relative contribution of intermediate nucleon resonances to the final state. However nucleon resonances with masses around 1.710 $${\textrm{GeV}/\textrm{c}^{2}}$$ GeV / c 2 ( $${\textrm{N}^{*}(1710)}$$ N ∗ ( 1710 ) ) and 1.900 $${\textrm{GeV}/\textrm{c}^{2}}$$ GeV / c 2 ( $${\textrm{N}^{*}(1900)}$$ N ∗ ( 1900 ) or $${\Delta ^{*}(1900)}$$ Δ ∗ ( 1900 ) ) are preferred by the fit.
The production of
Σ
0
hyperons in proton proton collisions at a beam kinetic energy of 3.5 GeV impinging on a liquid hydrogen target was investigated using data collected with the HADES setup. The ...total production cross section is found to be
σ
(
p
K
+
Σ
0
)
=
17.7
±
1.7
(
s
t
a
t
)
±
1.6
(
s
y
s
t
)
µb. Differential cross section distributions of the exclusive channel
p
p
→
p
K
+
Σ
0
were analyzed in the center-of-mass, Gottfried–Jackson and helicity reference frames for the first time at the excess energy of 556 MeV. The data support the interplay between pion and kaon exchange mechanisms and clearly demonstrate the contribution of interfering nucleon resonances decaying to
K
+
Σ
0
. The Bonn–Gatchina partial wave analysis was employed to analyse the data. Due to the limited statistics, it was not possible to obtain an unambiguous determination of the relative contribution of intermediate nucleon resonances to the final state. However nucleon resonances with masses around 1.710
GeV
/
c
2
(
N
∗
(
1710
)
) and 1.900
GeV
/
c
2
(
N
∗
(
1900
)
or
Δ
∗
(
1900
)
) are preferred by the fit.
High-statistics
π
-
π
-
and
π
+
π
+
femtoscopy data are presented for Au + Au collisions at
s
NN
=
2.4
GeV
, measured with HADES at SIS18/GSI. The experimental correlation functions allow the ...determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the cube root of the number of participants. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.
Abstract
High-precision measurements of flow coefficients
$$v_{n}$$
v
n
(
$$n = 1 - 4$$
n
=
1
-
4
) for protons, deuterons and tritons relative to the first-order spectator plane have been performed ...in Au+Au collisions at
$$\sqrt{s_{_{{\text {NN}}}= 2.4$$
s
NN
=
2.4
GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentum
$$p_{{\text {t}}}$$
p
t
and rapidity
$$y_{{\text {cm}}}$$
y
cm
over a large region of phase-space and for several classes of collision centrality. A clear mass hierarchy, as expected by relativistic hydrodynamics, is found for the slope of
$$v_{1}$$
v
1
,
$$d v_{1}/d y^{\prime }|_{y^{\prime } = 0}$$
d
v
1
/
d
y
′
|
y
′
=
0
where
$$y^{\prime }$$
y
′
is the scaled rapidity, and for
$$v_{2}$$
v
2
at mid-rapidity. Scaling with the number of nucleons is observed for the
$$p_{{\text {t}}}$$
p
t
dependence of
$$v_{2}$$
v
2
and
$$v_{4}$$
v
4
at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.
$$v_{2}$$
v
2
is found to scale with the initial eccentricity
$$\langle \epsilon _{2} \rangle $$
⟨
ϵ
2
⟩
, while
$$v_{4}$$
v
4
scales with
$$\langle \epsilon _{2} \rangle ^{2}$$
⟨
ϵ
2
⟩
2
and
$$\langle \epsilon _{4} \rangle $$
⟨
ϵ
4
⟩
. The multi-differential high-precision data on
$$v_{1}$$
v
1
,
$$v_{2}$$
v
2
,
$$v_{3}$$
v
3
, and
$$v_{4}$$
v
4
provides important constraints on the equation-of-state of compressed baryonic matter.
In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as ...well as on pion–pion momentum correlations is investigated in Au + Au collisions at
s
NN
= 2.4 GeV. We show that the low-
m
t
region (
m
t
<
0.2
GeV/
c
2
) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a
A
part
2
/
3
scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.
We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p ...reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e−>0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.
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